Along with Gil Robinov, an executive managing director at NAI Global who has represented about 25 percent of U.N. member governments, no other broker in New York City is as knowledgeable as Mr. Roos when it comes to the tricky property needs encompassing United Nations Plaza.

Indeed, as the exclusive agent for the United Nations, the longtime Colliers International vice chairman has been advising the body on its real estate interests for more than 20 years. And, yet, as he told The Commercial Observer, “I’ve had a pretty intense run with them over the last few years or so.”

Relocating offices amid a renovation of the Secretariat building as part of a U.N. capital plan, finding new space for the United Nations Population Fund, and leasing 50,000 square feet of space at 220 East 42nd Street to the United Nations Development Program are just a few of the assignments that Mr. Roos has been involved with over the past six months.

Because it has diplomatic immunity shielding it from disputes in court, the U.N. has traditionally been a tough tenant to sell to prospective landlords. As a result, Mr. Roos and his Colliers International team created a standard lease for the body unlike anything regular tenants will ever see.

“We’ve been successful in the last few years in changing attitudes,” said Mr. Roos, 57, whose family members have held top spots at the firm since it was first launched as Williams, in 1926. In the decades between then and today, the firm has taken several new names, including GVA Williams and Colliers International.

“Before the downturn in the market, there were a lot of landlords who just didn’t feel the need to have the United Nations as a tenant because of all the immunity issues and some incorrect perceptions,” Mr. Roos added. “That mind-set has changed significantly.”

Mr. Roos, who has completed upward of 30 million square feet in deals throughout his 30-year career, has also inked transactions for a slew of other non-U.N. clients. A couple of recent examples: the law firm Holland + Knight, which took space at 40 West 52nd Street last year, and a relocation of headquarters space for the Xerox Corporation at 245 Park Avenue.

Throughout his career, the broker has been acutely aware of how creativity can benefit both landlord and tenants.

“I became a late bloomer in the sense that there were certain life events that encouraged me to allow my creativity to dictate how I went about business,” said Mr. Roos, a former photographer whose sister, he said, died relatively young. “My career has been defined by creativity more than aggression. I’m not a scorched-earth guy. I really think that it’s not always about winning.”

IT WAS PARTIALLY PLUCK, or perhaps even another’s misfortune, that propelled the ribbon-cutter Sydney Roos to the upper tiers of Williams, the seminal New York City real estate firm whose origins can be traced back 80-odd years.

Shortly after he joined in 1929, in fact, the principal after whom the firm was named-William Silverman-suffered a nervous breakdown before agreeing to sell a cut of the business to partner Victor Cohen. Being the company’s top salesman, Sydney Roos was a no-brainer to join Mr. Cohen as the new principal.

More than a decade passed before Roos’ son strayed, too, from his career path, side-winding from an engineering job to a sales position at Williams. Like his father, Edwin Roos also partnered with a Cohen-Victor’s son, Jerome.

It was after a stint as a photojournalist for the British music magazine Melody Maker that the shaggy-haired Andrew Roos stepped away from the Jackson Browne and Joni Mitchell acts he was photographing for a stab at the family business. He, too, went to Williams, taking Michael Cohen as his partner.As the third-generation member describes it, the union forged between the Roos and Cohen clans has sprung much more from common sense than out of some loyalty to tradition. In other words, if it’s not broke, don’t fix it.

“It was totally organic,” Mr. Roos said of the camaraderie with the Cohen family and, even more specifically, his partner. “Michael and I had very little contact growing up-he lived in the city and I was over in Long Island. It was a little bit like a shotgun wedding, but it’s all worked out fine.”

Now the father of three children, with a wife of 30 years and a home in Westchester, Mr. Roos still fiddles with his cameras when he isn’t riding horses at his ranch, racing speedsters or skiing.

“I’m very lucky,” said Mr. Roos, who studied at American University and the University of Colorado. “I’ve got a balanced life and great kids. And I’ve got a wife I adore and other outside interests. So, as I said, I’m very lucky.”

As for his father, for whom he never worked directly, Mr. Roos was slightly more introspective. When asked what advice the now-retired patriarch had given, he first deadpanned “plastics” before discarding the famous quote from The Graduate for what could be described as a tribute to his family.

“You know, I look back and, honestly … honesty, integrity and also following through on what you say you’re going to do,” said Mr. Roos, before shifting the focus to his father’s father. “My grandfather was this larger-than-life figure, and I’m not sure how much he really knew about real estate. But he knew about people. And that’s guided how I’ve approached my career.”